1. Field of the Invention
This invention relates to a thermal flow sensor, especially a hot film type flow sensor comprising a heating resistor and a fluid temperature sensing resistor, both of which are composed of a thin film on an insulating substrate.
2. Description of the Prior Art
As a thermal flow sensor, a flow meter in which the main flow tube is provided with a by-pass tube through which a portion of the main flow is directed and to which a heater is connected is widely used. This flow meter detects the flow rate from the distribution of heat that arises when fluid flows through the by-pass tube while the by-pass tube is heated. Such a flow meter has such excellent accuracy that it is widely used for the control of the flow rate of semiconductor gases, etc., but it is not suited for miniaturization and/or mass production. Moreover, its production cost is so high that it can only be applied to a limited field.
As another thermal flow sensor, there is a hot wire type flow sensor comprising a heating resistor (called a hot wire) and a fluid temperature sensing resistor (called a cold wire), by which the flow rate is calculated based on changes in the amount of heat transferred from the heating resistor to the fluid surrounding this heating resistor. Taking into account the temperature of the fluid detected by the fluid temperature sensing resistor, the difference in temperature between the fluid and the heating resistor is maintained at a fixed value so that compensation for changes in the fluid temperature can be made and a quick response can be obtained regardless of the heat capacity of the heating resistor. A wire of platinum, tungsten, etc., is used in the heating resistor and the fluid temperature sensing resistor. However, the resistance of the wire is small and the resistance among flow sensors is greatly scattered, so that adjustability of the heating temperature and accuracy of the temperature measurement are poor. Moreover, thin wire is used, so that manufacture is difficult and mass production is not possible.
As another thermal flow sensor, there is a hot film type flow sensor in which a thin metal film patterned on an insulating substrate is used instead of the wire of the above-mentioned hot wire type flow sensor. Due to the use of a fine patterned thin metal film, the flow sensor can be miniaturized. Moreover, since a number of units can be disposed on a single substrate, mass-production is possible and little scatter arises. Thus, this kind of flow sensor is actively being studied.
There is also a flow sensor using a heating diffusion-resistor (or transistor) and a fluid temperature sensing diffusion-resistor (or transistor) both of which are built on a silicon chip. This flow sensor is manufactured by the use of a silicon processing technique, so that it is readily mass-produced, but the temperature characteristics of the sensor are varied, resulting in difficulties of the establishment of a high heating temperature.
The hot film type flow sensor is composed of a heating resistor and a fluid temperature sensing resistor such as those of the hot wire type flow sensor, and the operation principle of the hot film type flow sensor is the same as that of the hot wire type flow sensor, which is represented by the following equation (1): ##EQU1## wherein I is the current passing through the heating resistor, Rh is the resistance of the heating resistor, Th is the temperature of the heating resistor, Ta is the fluid temperature (i.e., the temperature of the fluid temperature sensing resistor), U is the flow rate of the fluid, and A and B, respectively, are a fixed value.
Current is, first, supplied to the heating resistor, resulting in heat generation in the heating resistor. When the flow rate of the fluid to be measured is high, a large amount of heat is transferred from the heating resistor to the fluid. On the contrary, when the flow rate of the fluid is low, a small amount of heat is transferred from the heating resistor to the fluid. Thus, the flow rate of the fluid can be determined by detecting changes in the amount of heat transferred from the heating resistor to the fluid in the following manner: Current that is applied to the heating resistor is maintained at a fixed level and the temperatures of both the heating resistor and the fluid are measured while the fluid flows. Alternatively, the difference in temperature between the fluid and the heating resistor is maintained at a fixed level by the control of current that is applied to the heating resistor, and the flow rate of the fluid can be calculated from changes in the current. Usually, because of a quick response, the latter process, in which the difference in temperature between the fluid and the heating resistor is maintained at a fixed level is used.
Even when either of the above-mentioned two processes is used, the temperature Ta of the fluid and the temperature Th of the heating resistor must be continuously measured. For this purpose, a fluid temperature sensing resistor for detecting the temperature of the fluid in addition to the temperature of the heating resistor is required. In order to detect the temperatures of both the fluid and the heating resistor with accuracy, the heating resistor and the fluid temperature sensing resistor must be thermally isolated from each other. Therefore, conventional thermal flow sensors must be provided with a structure that the heating resistor and a fluid temperature sensing resistor, respectively, are disposed on separate plates, so that these conventional thermal flow sensors become bulky and are not suitable for mass-production.